A study of void formation in fast neutron-irradiated metals

Abstract

Experimental observations on the temperature dependence of void formation in Type 304 stainless steel are presented. The average void size is seen to increase while the void number density decreases with increasing irradiation temperature. The total void volume reaches a maximum at an intermediate temperature (∼500°C). A thermodynamically based model for void formation has been developed in an effort to understand these and other experimental observations. The model is a quasi-steady-state approach that attempts to describe the void and dislocation loop nucleation and growth rates as a function of temperature, neutron flux, and type and density of point-defect sinks. The model was written in the form of a computer routine which iterates over small time steps and fitted to experimental observations of void formation in irradiated austenitic stainless steel by adjustment of material constants within reasonable limits. Good agreement was obtained for a wide range of experimental results. Development of the model has emphasized that some kind of preferential attraction between one type of sink and one type of point defect is necessary in order for void growth to occur. The preferential attraction suggested is that between the stress field of a dislocation and the misfit strain of an interstitial. The model also pointed out the necessity of separating the neutron flux and time of irradiation when reporting information on void formation. It is expected that quite different dependences of void size and number density on the total neutron dose would be obtained depending on whether the experimental information was obtained under constant flux or under constant time conditions. It was also concluded that the magnitude and temperature dependence of dislocation loop formation would have a marked effect on void formation.